Travelling wave tubes



1958 D. c. ROGERS ET AL 2,824,996

TRAVELLING WAVE TUBES Filed Aug. 2, 1954 Inventors D. C. ROGERS- F. F. C. BURKE Attorney rates TRAvELLnsG WAVE TUBES Application August 2, 1954, Serial No. 447,173

Claims priority, application Great Britain October 14, 1953 Claims. (Cl. 3153.5)

Unite The present invention relates to travelling wave tubes.

In a travelling wave tube a beam of electrons is caused to interact with electromagnetic waves guided by a waveguide structure in such manner that a component of the waves travels along the electron beam path with a phase velocity approximately the same as that of the electrons. In the kind of tube most commonly used today the aforementioned waveguide structure is provided by a conductor formed into a helix. The travelling wave tube is most commonly used as a broad band amplifier but it is also capable of use as an oscillator or as a frequency changing device, or, indeed, in a number of ways in circuits analogous to those for which, at lower frequencies, triode and multi-grid thermionic valves are used. In association with the helix of a travelling wave tube, therefore, there must be at least an output wave coupling and, for most applications, both input and output connections. For this purpose the travelling wave' tube usually projects through a rectangular hollow waveguide or through a cavity built out from a coaxial transmission line; in the present specification such a built-out transmission line is considered to be included in the term hollow waveguide. While it is desired that electromagnetic waves may pass from the input waveguide to the helix or from the helix to the output waveguide, means have to be provided for preventing escape around the outside of the travelling wave tube of electromagnetic waves from the apertures in the waveguide walls through which the tube projects. The apparatus, therefore, in-

cludes a waveguide choke formed by a cylindrical flange projecting from the wall of the waveguide around the travelling wave tube in the direction opposite to the helix, I

and, inside the tube envelope, a corresponding inner sleeve member, coaxial with the cylindrical flange, thus providing, during operation, capacitative coupling between the waveguide or cavity wall and the internal sleeve member. The two choke members are usually a quarter wavelength long at the mean operating frequency, but, provided the characteristic impedance of the coaxial line formed between the inner and outer choke members is low, the exact length is unimportant.

The helix must be accurately dimensioned and must be adequately supported in the travelling wave tube so that it shall not intercept the electron beam current. Although the envelope of the travelling wave tube may itself provide the means of supporting and aligning the helix, it is generally preferred to use independent supporting means, such, for example, as glass rods arranged longitudinally about the exterior of the helix, and resting against the inner wall of the glass envelope. In one known type of travelling wave tube these glass rods are themselves seated in appropriate recesses in the internal waveguide choke members.

According to the present invention there is provided a travelling wave tube of the kind in which during operation a beam of electrons is projected along a waveguide structure in the form of a helix coupled by means includ 2,824,996 6 Patented Feb; 25, less ing a Waveguide choke to a hollow waveguide and interchanges energy with electromagnetic waves propagated along the said helix with a phase velocity approximately that of the said electrons, the said travelling wave tube comprising an electron gun structure, an electron collector electrode and a tube of insulating material supporting the said helix within an outer envelope and mounted at each end upon a sleeve member, at least one of which forms. part of a said waveguide choke, the said sleeve member at the electron gun end of the tube being mounted upon or forming part of the said electron gun structure.

The invention will now be described with reference to the accompanying drawings, in which:

.Fig. 1 shows diagrammatically the essential parts of a travelling wave tube according to the invention and associated waveguide apparatus with which the present invention is concerned; and

Fig. 2 shows a similar view of another embodiment of the invention.

In Fig. 1, which, primarily because of the very different thickness of material involved, the various parts are not shown necessarily to scale, a travelling wave tube, of which only the ends are shown, is indicated at 1 and comprises, at one end, an envelope bulb 2, housing an electron gun 3, and at the other end a collector electrode 4 sealed to the end of an envelope portion 5 of smaller diameter and considerably longer than the bulb portion 2. The envelope portion 5 surrounds a wire helix 6 supported within a tube of insulated material 7 which is mounted at either end upon respective sleeve members 8 and 9. To provide atenuation in association with the helix waveguide structure, some lossy material can be painted or evaporated on the inside of tube 7, or, if the tube is sufficiently thin, on the outside. The envelope portion 5 is shown projecting through input and output hollow waveguides 10 and 11, respectively, which carry extensions 12 and 13 shown fitted with adjustable pistons 14 for improving the impedance match between the respective waveguides and the helix. Each of the waveguides is provided with a cylindrical flange 15 surround ing the travelling wave tube and projecting away from the helix, These flanges form, with the respective sleeves 8 and 9, waveguide chokes preventing escape of electromagnetic wave energy towards the electron gun and to- Wards the collector electrode respectively. In the form shown in Fig. 1 each of the sleeve members 8 and 9 is formed by a disc portion 16 integral with a cylindrical skirt portion 17; member 8 carries a central tubular boss 18 projecting to either side of the disc portion 16 and member 9 a metallic tube 19 projecting considerably to the rear of the skirt and projecting in front of the disc portion 16 similarly to boss 18. The boss 18 and front projection of tube 19 are connected by straight lengths of wire 20 to the respective ends of the helix 6 to form antenna probes for coupling between the helix and the respective waveguide. For supporting the sleeve member 8 and providing electrical connection to the helix, a metal tube 21 is secured atone end to the final anode 22 of the electron gun 3, and fits snugly at its other end over the rear portion of the boss 18. Besides the anode 21 the electron gun 3 comprises a cathode 23 surrounded by a focusing cylinder 24, the electrodes being supported by means such as support rods 25 to respective insulating washers, the end one of which is shown at 26. The tube 21 can thus be secured, if desired, to the insulator 26 instead of to the anode 22; an independent connection for the helix can then be provided.

At the other end of the tube the electron collector electrode 4, which is represented as sealed to the envelope portion 5 by a feather-edged portion 27, encloses a generally cylindrical cavity 28' in which the electrons of the beam are collected. The wall of cavity 28 includes a a step, indicatedvat 29,'against which is seated a tube of insulating material closely surrounding the rearward projection: of tube 19. The tube 'thus 'supports th'e output end of thef helix and sleeve assembly-. "Theap-ro vision, in sleevemember9, of the tube 191a place of the shorter boss 18 of sleeve member SpreVeBts-the col lection of charges on the inner wall'of the tu-betlsth which would, otherwise directly surround the electron beam;

ltwill beapprecia'ted'that, in the co'nstructionof Fig;

1, the electron gun structure, the helix and'its'supporting tube mounted: upon the sleeve members 8 and 9 together with. the sleeve supportitubes121 and 39; can be constructed as a n'gid unitary assembly which may a be inserted at a convenient stage of manufacture into tii'e travelling. wave tube envelope and pushed home until the insulating tube fiti seats-against the step 29; thus ensuring acciirate'ailignrnent and axial location of the-el'ecmates correctly withflthe collector electrode 4.- The electron beam has alsoto travel a. considerabledistancc from the final anode 22 to the commencement of the V helix, so making the apparatus considerably longer than the effective working part between the waveguides Iii} and 11.' (The apparatus external to'the travelling wave tube, besides the input and output :waveguide'shown,

comprises an assembly of magnetic focussing COliS-flildl means for supporting the travelling wave tube.) In the tube of Fig. 2, the external envelope 3Z'is a simple tubeof glass, of relatively uncritical dimensions, forming a hermetic enclosure between a pair of end caps 33a'nd-34' between which the electrode structure of the tube is supported independently of the envelope 32. The general type of 'constmction of this tube forms the subject-matter of co-pending application of D. C. Rogers-P. F. C. Burke Serial No. 448,816, filedAugust-8, 1954.- The tube'has various other advantagesand novel features of construction, but in the present specification only such parts of the tube and its associated apparatus willbe described as is necessary for an understanding of the application of the present invention thereto". V j i An electron'gun structure 35 is secured to the end'cap;

33' by means of a cylindrical skirt 36; The gun structure 35' comprises a hollow cylindrical outer member of' magnetic material; which is 'the only part visible'in the drawing of' Fig. 2, closed at the end remote from the skirt 35; and the said outer member comprises, in* itself, part of" the final anode structuregfof the; electron gun system, the actual anode being a copper insertin the front face 37'. 1 This. eylindrical outer'member' al'so provides a magnetic shield for the electron gun system, one of the pole pieces for producing a magnetic fieldalong the axis of the electron beam, and an inner waveg'uide choke sleeve memberanalogous to the member 8 of Fig.1. At the other end of the travelling wave tube theend cap 34 has secured 'to it an electron collector electrode 38,- which' is shown provided with cooling fins 39; andametal tube 40, through which the envelope -32 is evacuated; j

and which we shall refer to as pumping tube. V The pump; ing tube provides thesupport at this end of the travelling wave tube for aunitary electrode assembly which com-f prisesthe remaining electrodes secured to--the electron gun structure 35. This unitary assemblyterminates; at the electron coliector end of the'tube, in asleeve member.

5; of ferromagnetic material, which forms,;n0t only an inner quarterewave-sleeve forfa. waveguide. choke? associated with the output waveguide of. the apparatus, but:

2,824,996 r a i a also the other pole piece. for the aforementioned axial magnetic focussing field. From the aspecto'f the present invention the electron gun structure 35 and the member 41 are directly analogous to the sleeve members Sand of Fig. l and carry between them a central 'tube of insulating material 42 having -a-fprecision bore supported on tubular bosses 43 and 44 projecting respectively from the front face '37 of the electron gun structure 35iand L from the front face of the sleeve member 41 The lielix V 45, instead or". terminating at either endin 'straightlengths of wire forming probe antennae asa't20, 'l are ere brought straight out of the support tube eZthrough slots 46 and are connected to respective generally U-shaped probe membersv 47, respectively securectto; andiprojecting from, the face 37 of the ele cti-on guri structure 35 and the opposing face of sleeve memberAl. The type of probe member here shown is described and. claimed in an application of D. C. RogersSerial No. 407,732,]filed February 2} 1954 The electron-gun structure 35 'a1nd the sleeve member 41 areseeured' togetherbymeansiof three ceramic rods, or which twoare indicated M348;

These rods 48 are fitted at theirends with sleeves having circular flanges 49, which a-reweldedto the 'res peetive" 0 Using surfaces of the electron un structure'35 'an'd3 I: a t

sleeve member 41. i

In operation the travelling wave tube 31' is V v through input waveguide 50 and output WaVegui d e S'L and is positionedaxially so that the opposing faces of the electron gun structure 35 and the sleeve'm ember '41 arealigned with the respective edges" of the internal walls of" waveguides 56 and 51.1 In place of theflange 15" of Fig. 1 an outer cylinder of ferromagnetic material 52-su rrounds theelectron gun-structure and butts agai'nst'the This. cylinder'f orms part of the external magnetic circuit providing the focus- 7 Outer wall of the input waveguide.

ing fieldreferred to above and also forms the outerwaveguide choke member, co-operating with the gun struc- V ture 35, toprevent leakage of electromagnetic waveenergy" from the waveguide 45 in the direction ofthe end capf'33 as it then effectively forms part of the outer wall ofihe.

waveguide it is copper-plated; Themember'35 is not; in

this case a quarter wave length: long, but ratheriis ccn; siderabl-y more. The cylinder 52 has'therefore preferably; a section of small internal, diametera quarter wave lengthvlong abutting the waveguide and a longer section 53 of substantially larger bore. Thus the quarter wave choke: is terminated in a line of considerably higher characteristic impedance which corresponds sufiiciently to an open circuit-to ensure that little R; Fi powerflcws towards the end cap 33 It may be observed thatffi'onr the magnetic circuit point of view theelectron gun structure 35 projects beyond'the end of'the cylinder 52 by the thickness of the waveguide wall. If-desirecL'the waveguide wall could be cut away so as to allow the cylinder52 to bemounted flush with'theinner surface of the waveguide rather than butting against the outer wall. On the other hand-, we have found that, if anything, there is some advantage in having the inner pole piece member formed by the gun structure 35 projecting slightly' beyond the cylinder 52 At the other end of the travelling wave tube'a cylinder 54; somewhat" shorter than52, but likewise of copper- I plated ferromagnetic material; butts against the outer. wallof waveguide 51. This member 'forms theqother; outer pole piece of the magnetic circuitproviding the. axial magnetiefocussing field for the electronbeam, so that the two members 41 and 54 togetherform tati thee' same time, one of the poles of the magnetic circuit and also a waveguidechoke preventing escape of energy from f the waveguide 51 in the direction-of the end] eap- 342 'Inthis case, however, it"is convenient to make thefmemberf 41 a" quarter wavelength long at" the .mearn operating fre'quencyttakin'g; into. accountithet eflect of the: filial?" veiling dielectric provided by the: glass-enveloper3i2.) so

that the members; do form at quarterrwave: choke.

" Inserted;

Because of the large diameter of the envelope 32, as compared with the envelope portion 5 of Fig. 1, the apertures in the waveguide walls surrounding the helix 45 are too large to be ignored; consequently the arrangement of Fig. 2 includes additional waveguide chokes in the form of quarter-wave sleeve members 55 and 56, cooperating with respective flanges 57 and 58, extending from the waveguide walls and surrounding the travelling wave tube between the two waveguides. For reasons of space, adjustable matching sections such as 12 and 13 of Fig. 1 with their pistons 14 are not provided in the arrangement of Fig. 2, for the piston would then foul the larger envelope 32. Instead, closed matching sections 59 and 60, respectively, are provided in continuation of the waveguides 49 and 5t), and at each end of the travelling wave tube, pairs of short circuiting rods, one rod being visible at either end at 61 and 62, are positioned within the tube envelope above the extensions 59 and 60, respectively. The short circuiting rods 61 are let in to the surface 37 of electron gun structure 35 and are secured in the sleeve member 55, at the same time providing electrical connections thereto and mechanical support for the sleeve. Similar arrangements obtain at the other end of the tube, the pair of shorting rods 62 joining and supporting the sleeve member 56. The sleeve members 55 and 56 are generally similar in construction to the sleeve members 8 and 9 of Fig. l but are apertured to permit passage of the ceramic rods 47 therethrough.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What we claim is:

1. A travelling wave tube of the kind in which, during operation, a beam of electrons is projected along a waveguide structure in the form of a helical coil coupled by means including waveguide chokes to a hollow waveguide and interchanges energy with electromagnetic waves propagated along the said helical coil with a phase velocity approximately that of the said electrons, the said travelling wave tube comprising an electron gun structure, and an electron collector electrode, an assembly comprising a cylindrical tube of insulating material supporting said helical coil tubular sleeve members supporting said tubular member, means connecting said helical coil to said tubular members, said members being positioned to form part of said waveguide chokes, means for rigidly connecting said electron gun structure to one of said tubular members, means for rigidly conecting said electron collector to the other of said tubular members, and a sealed outer envelope external of the tube assembly.

2. A travelling wave tube comprising: an electron gun for producing an electron beam; a helical coil of conducting material supported inside a tube of insulating material; a pair of waveguide choke inner sleeve members, each supporting one end of the said tube of insulating material; an outer envelope surrounding the said electron gun, the said tube of insulating material and the said sleeve members; an electron collector electrode closing the end of the said envelope opposite the electron gun; a tube of conducting material surrounding the electron beam secured to the structure of the electron gun and to the adjacent said sleeve member; and tubular supporting means surrounding the electron beam and secured to the other said sleevemember seated within the said electron collector electrode in such manner that the said sleeve member and the said electron collector electrode are insulated one from the other.

3. A travelling wave tube according to claim 2 in which the said tubular supporting means comprises a tube of insulating material engaging with the said electron collector electrode and having a conductive lining.

4. A travelling wave tube according to claim 1 in which the said sleeve members are each formed as a metal disc having a cylindrical skirt and an annular central boss which is inserted in the end of the helix supporting tube.

5. A travelling Wave tube according to claim 1 wherein said means for connecting said helical coils each comprise a waveguide probe antenna secured to the corresponding sleeve member and joined to the adjacent end of the said helical coil, said cylindrical tube being slotted at this end for passage of the connection between the helix and the said probe antenna.

References Cited in the file of this patent UNITED STATES PATENTS 2,602,148 Pierce July 1, 1952 2,611,102 Bohlke Sept. 16, 1952 2,672,571 Harman Mar. 16, 1954 2,749,472 Field June 5, 1956 2,774,006 Field et a1. Dec. 11, 1956 

